Remember the problem about a negative or too large "repeat" value in Section 2, “Other types of options”? The code that prints the greeting has those lines:
...
repeat = cfg_getint(cfg_greet, "repeat");
while(repeat--)
...
The repeat variable is defined as an int, a signed integer. If the user specified a negative repeat value in the configuration file, this code would continue to decrease the repeat variable until it eventually underflowed.
We'll fix this by not allowing a negative value in the configuration file. Of course we could first just check if the value is negative and then abort, using cfg_getint() and a test. But we will use a validating callback function instead. This way cfg_parse() will return an error directly when parsing the file, additionally indicating on which line the error is.
A validating callback function is defined as:
typedef int (*cfg_validate_callback_t)(cfg_t *cfg, cfg_opt_t *opt);
This function takes two arguments: the section and the option. It should return 0 on success (ie, the value validated ok). All other values indicates an error, and the parsing is aborted. The callback function should notify the error itself, for example by calling cfg_error().
Here is the code for the callback function:
01 int validate_unsigned_int(cfg_t *cfg, cfg_opt_t *opt)
02 {
03 int value = cfg_opt_getnint(opt, cfg_opt_size(opt) - 1);
04 if(value < 0)
05 {
06 cfg_error(cfg, "integer option '%s' must be positive in section '%s'",
07 opt->name, cfg->name);
08 return -1;
09 }
10 return 0;
11 }
12
Only the last value is validated, because libConfuse will call this function once for every value corresponding to the option. Since the "repeat" option is not a list, we could instead have used cfg_opt_getint(opt) to retrieve the only value. However, if we later want to use this callback to validate an integer list, it is already lists-aware.
The validating callback is installed with cfg_set_validate_func(). It is called with a string specifying which option is affected, and a pointer to the callback function. To specify an option in a subsection, the section and the option must be separated with a vertical bar ("|").
We're now also looking at the return code from cfg_parse() to verify that the parsing was successful. The complete program is now:
01 #include <stdio.h>
02 #include <confuse.h>
03
04 int validate_unsigned_int(cfg_t *cfg, cfg_opt_t *opt)
05 {
06 int value = cfg_opt_getnint(opt, cfg_opt_size(opt) - 1);
07 if(value < 0)
08 {
09 cfg_error(cfg, "integer option '%s' must be positive in section '%s'",
10 opt->name, cfg->name);
11 return -1;
12 }
13 return 0;
14 }
15
16 int main(void)
17 {
18 cfg_opt_t greet_opts[] =
19 {
20 CFG_STR_LIST("targets", "{World}", CFGF_NONE),
21 CFG_INT("repeat", 1, CFGF_NONE),
22 CFG_END()
23 };
24 cfg_opt_t opts[] =
25 {
26 CFG_SEC("greeting", greet_opts, CFGF_TITLE | CFGF_MULTI),
27 CFG_END()
28 };
29 cfg_t *cfg, *cfg_greet;
30 int repeat;
31 int i, j;
32
33 cfg = cfg_init(opts, CFGF_NONE);
34 cfg_set_validate_func(cfg, "greeting|repeat", validate_unsigned_int);
35 if(cfg_parse(cfg, "hello.conf") == CFG_PARSE_ERROR)
36 return 1;
37
38 for(j = 0; j < cfg_size(cfg, "greeting"); j++)
39 {
40 cfg_greet = cfg_getnsec(cfg, "greeting", j);
41
42 repeat = cfg_getint(cfg_greet, "repeat");
43 while(repeat--)
44 {
45 printf("%s", cfg_title(cfg_greet));
46 for(i = 0; i < cfg_size(cfg_greet, "targets"); i++)
47 printf(", %s", cfg_getnstr(cfg_greet, "targets", i));
48 printf("!\n");
49 }
50 }
51
52 cfg_free(cfg);
53 return 0;
54 }
55